**3.4 Functionalization of the silica spheres**

In order to create open links over the silica to attach the GNPs, the silica was functionalized with APTES on a 1 ml:1 μl silica/APTES volume ratio. About 50 ml

### **Figure 1.**

*Images of the synthesis of SiO2 particles: (a) right after adding TEOS, (b) at 10 min of reaction, (c) at 30 min of reaction, and (d) at 2 h of reaction.*

of the silica template solution was magnetically stirred for 5 min with 50 μl of APTES in an 80 ml glass flask. The solution was left still overnight at room temperature. The opaque white functionalized silica particles precipitated in the solution leaving a clear fluid at the top. To separate the functionalized silica, the mixture was centrifuged at 6000 rpm for 1.5 min and washed in deionized water three times. Finally, they were sonicated in 20 ml of deionized water final volume.
